Pharmacology - from Katzung's chapter 2 - Pharmacokinetics & Pharmacodynamics

Question 1

Your patient has been receiving an IV infusion of lidocaine at 2mg/min for 8hrs. There are now some signs of toxicity. You obtain a plasma concentration and find that it is 10mg/L, which is above the therapeutic window, and twice the target concentration. How should the infusion rate be modified? The half-life is 1.4hrs.

A) Change to 1mg/min
B) Change to 0.5mg/min
C) Halted for 1 half-life (1.4hrs), then restarted at 2mg/min
D) Halted for 1 half-life (1.4hrs), then restarted at 1mg/min

Answer 1

D.

This means that the patient’s clearance must be lower than expected, e.g. there may be renal disease.

Question 2

A 74-year-old retired mechanic is admitted with a myocardial infarction and severe acute cardiac arrhythmia. You decide to give a lidocaine infusion to correct the arrhythmia. A continuous IV infusion of 2mg/min is started. Assume the following: Vd = 80L. Clearance = 400mL/min. What is the expected steady-state concentration?

A) 3mg/L
B) 4mg/L
C) 5mg/L
D) 9mg/L
E) 15mg/L

Answer 2

C.

This requires the equation:
“Rate in” = Rate of Elimination (because it’s steady state)
Rate of Elimination = Clearance x Concentration
so:
“Rate in” = Clearance x Target Concentration